Hydraulic elevators are commonly used instead of traction type elevators in low rise applications. The advantage of the hydraulic elevator is its lower cost. This advantage, however, may be offset by the lack of precision control of the ride as compared to traction elevators.
In a typical hydraulic elevator, the flow of fluid to and from a hydraulic cylinder causes the elevator car to ascend and descend within the hoistway. During the ascent operation, the fluid is pumped from a tank by a pump and flows through a control valve before entering the cylinder. During the descent operation, the control valve opens to permit the fluid to flow from the cylinder and into the tank under the pressure of the car. The motion profile of the elevator includes an acceleration phase, a full speed phase, a deceleration phase, and a leveling phase. In the leveling phase, the position of the elevator is corrected to level it with the landing. The leveling phase increases the flight time and the amount of work required of the hydraulic system and therefore it is desirable to minimize the leveling phase.
One type of hydraulic valve commonly used to transition between full speed and stopping, i.e., the deceleration phase of the elevator motion profile, includes a valve stem actuated by a solenoid to direct the flow of fluid through the valve. In this type of valve, the solenoid is activated to permit a flow of fluid into a cylinder closed off by a piston. Upon sufficicent pressurization of the cylinder, the piston will open to permit fluid flow through the valve and the car to descend. Balancing the fluid pressure on both sides of the valve prior to opening the valve provides a smooth and gentle start to the descent. A drawback to this type of valve is that the deceleration phase varies depending upon the hydraulic fluid viscosity and the static pressure in the hydraulic system (or the load on the hydraulic cylinder). This variation increases the amount of leveling necessary and, as a result, increases flight time, energy losses and the risk of overheating the hydraulic system.
A second type of transition valve includes a valve stem actuated by an electric motor. In this type of valve, flow is controlled by a stepper motor that moves a flow control valve. The amount of flow is programmed and controlled through feedback to produce a desired velocity profile for the elevator car. The purpose of the motor actuated control valve is to produce more precise control of the motion of the elevator car and thereby a smoother ride for the passengers that more closely approximates the ride of a traction type elevator. The main drawback to the motor actuated type of control valve is the additional complexity and cost associated with it.
The feedback to control the velocity profile may be open loop or closed loop. For open loop control, the hydraulic fluid temperature and static pressure are monitored and used to estimate a delay in carrying out the deceleration phase of the velocity profile. In this way, the control system attempts to compensate for viscosity variations of the hydraulic fluid. Since the delays are based upon predetermined estimates dependent upon parameters such as viscosity, the resulting performance of the control system in controlling the deceleration phase is less than optimal. For closed loop control of a motorized valve using car speed, high efficiencies can be achieved by constantly adjusting the valve position to approximate the desired velocity profile. The complexity of the control system and the added expense of this type of control may be prohibitive, however.
The above art notwithstanding, scientists and engineers under the direction of Applicant's Assignee are working to develop control systems to control the motion of elevators in a manner that optimizes flight time and efficiency without being cost prohibitive.